Kustomize

Kustomize introduces a template-free way to customize application configuration built into kubectl as apply -k.

https://kubectl.docs.kubernetes.io/guides/introduction/kustomize/

Extending

Kustomize offers a plugin framework allowing people to write their own resource generators and transformers.

https://kubectl.docs.kubernetes.io/guides/extending_kustomize/

Comparison to Helm

Helm is the package manager for Kubernetes.

https://harness.io/blog/helm-vs-kustomize/

Customizing Upstream Helm Charts with Kustomize

https://testingclouds.wordpress.com/2018/07/20/844/

Tanka

https://tanka.dev/

Manifesto

https://gitlab.com/jackatbancast/manifesto

CUE

https://cuelang.org/docs/about/